Recently, with practical implementation of a blue light-emitting diode (LED), development of a white LED using this blue LED is being aggressively pursued. The white LED ensures low power consumption and extended life compared with existing white light sources and therefore, its application to liquid crystal panel backlight, indoor or outdoor lighting device, and the like is expanding.
The white LED developed at present is obtained by coating a Ce-doped YAG (yttrium.aluminum.garnet) on the surface of blue LED. However, the fluorescence peak wavelength of Ce-doped YAG is in the vicinity of 530 nm and when this fluorescence color and light of blue LED are mixed to produce white light, the light is slightly blue-tinted. This kind of white LED has a problem of poor color rendering property.
To cope with this problem, many oxynitride phosphors are being studied and among others, an Eu-activated α-SiAlON phosphor is known to emit fluorescence (yellow-orange) with a peak wavelength of around 580 nm that is longer than the fluorescence peak wavelength of Ce-doped YAG (see, Patent Document 1). When a white LED is fabricated by using the α-SiAlON phosphor above or by combining it with a Ce-doped YAG phosphor, a white LED giving an incandescent color at a lower color temperature than a white LED using only Ce-doped YAG can be produced.
However, as a Ca-containing α-SiAlON phosphor activated with Eu, represented by the formula:CaxEuySi12−(m+n)Al(m+n)OnN16−n, a phosphor having a high luminance enough for practical use has not been developed yet.
Patent Document 2 discloses a phosphor exhibiting excellent luminous efficiency and having a fluorescence peak at a wavelength of 595 nm or more, and a production method thereof, where a smooth-surface particle larger than ever before is obtained by adding a previously synthesized α-SiAlON powder as a seed crystal for grain growth to the raw material powder and a powder having a specific particle size is obtained from the synthesized powder without applying a pulverization treatment.
Specifically, an α-SiAlON phosphor which is an α-SiAlON phosphor (x+y=1.75, O/N=0.03) having a composition of (Ca1.67,Eu0.08)(Si,Al)12(O,N)16 and in which the peak wavelength of the fluorescence spectrum obtained by excitation with blue light of 455 nm is from 599 to 601 nm and the luminous efficiency (=external quantum efficiency=absorptivity×internal quantum efficiency) is from 61 to 63%, is disclosed.
However, in the document above, specific examples of a phosphor having a florescence peak wavelength of less than 599 nm and a phosphor of more than 601 nm, each having a luminous efficiency enough for practical use, are not illustrated.
Patent Document 3 discloses: a light-emitting device characterized by using a phosphor containing, as a main component, an α-SiAlON represented by the formula: (Caα,Euβ) (Si,Al)12(O,N)16 (provided that 1.5<α+β<2.2, 0<β<0.2 and O/N≦0.04) and having a specific surface area of 0.1 to 0.35 m2/g; a vehicle lighting device using the same; and a headlamp.
The document above discloses working examples of an α-SiAlON phosphor, where the peak wavelengths of the fluorescence spectrums obtained by excitation with blue light of 455 nm are 592, 598 and 600 nm and the luminous efficiencies (=external quantum efficiency) thereof are 61.0, 62.7, and 63.2%, respectively.
However, in the document above, specific examples of a phosphor having a fluorescence peak wavelength of less than 592 nm and a phosphor of more than 600 nm, each having a luminous efficiency enough for practical use, are not illustrated.
Patent Document 4 discloses a SiAlON phosphor having a specific property of emitting light with high luminance compared with conventional phosphors, where a metal compound mixture capable of composing a SiAlON phosphor through firing is fired in a specific temperature range in a gas having a specific pressure, pulverized to a specific particle diameter, and then subjected to classification and a heat treatment.
However, the matter specifically disclosed in the document is only the peak luminous intensity and since the peak luminous intensity varies depending on the measuring apparatus and measurement conditions, it is not known whether a luminous intensity high enough for practice use is obtained. Out of the phosphors specifically disclosed in the document, a SiAlON phosphor having a highest-intensity wavelength (=fluorescence peak wavelength) of 573 nm at best is a phosphor having a longest wavelength.